When it comes to security, Firefox is the best browser. Not only because of the way that browser is built and how fast all bugs were fixed in it, but also because of its add-ons. Security add-ons are very effective to detect viruses before download, stop any sort of malicious scripts that could be injected to your computer. It also advice you which site you should avoid in order to be away from any bad cookies that could come from it.
Dr.Web anti-virus checker
This add-on will put an end to your doubts. If you're hesitating whether to download a file found on the Internet or not. You have just to check it with this add-on if it's virus-free or if it's infected.
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NoScript
IT blocks Java and javascript form being executed only for trusted websites. It's very effective against XSS attacks (cross-site scripting attacks).
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Mcfee Site advisor
It allows you to identify potentially dangerous websites. Mcfee rely on a bot that download files from websites and check if there are any malware.. It uses a small bar in your browser that turns into green if the site is clean, Yellow: if you have to be careful and RED if the website is potentially dangerous.
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